This invention relates to a method and apparatus for controlling transmission power in CDMA mobile communications. More particularly, the invention relates to a transmission power control method and apparatus for comparing the error rate of receive data and a target error rate on the receiving side, controlling a target SIR and causing the transmitting side to control transmission power in such a manner that measured SIR will agree with the target SIR.
In order to distinguish a channel by a spreading code in W-CDMA mobile communications, multiple channels can share a single frequency band. In an actual mobile communications environment, however, a receive signal is susceptible to interference from its own channel and from other channels owing to delayed waves ascribable to multipath fading and radio waves from other cells, and this interference has an adverse influence upon channel separation. Further, the amount of interference sustained by a receive signal varies with time owing to momentary fluctuations in reception power ascribable to multipath fading and changes in the number of users communicating simultaneously. In an environment in which a receive signal is susceptible to noise that varies with time in this fashion, it is difficult for the quality of a receive signal in a mobile station linked to a base station to be maintained at a desired quality in a stable manner.
Inner-Loop Transmission Power Control
In order to follow up a change in number of interfering users and a momentary fluctuation caused by multipath fading, inner-loop transmission power control is carried out. In such control, the signal-to-interference ratio (SIR) is measured on the receiving side and the measured value is compared with a target SIR, whereby control is exercised in such a manner that the SIR on the receiving side will approach the target SIR.
FIG. 10 is a system block diagram for describing inner-loop transmission power control according to the prior art. Here only one channel of the system is illustrated. FIG. 11 is a flowchart of processing in inner-loop transmission power control according to the prior art.
A spread-spectrum modulator 1a of a base station 1 spread-spectrum modulates transmit data using a spreading code conforming to a specified channel. The spread-spectrum modulated signal is subjected to processing such as orthogonal modulation and frequency conversion and the resultant signal is input to a power amplifier 1b, which amplifies this signal and transmits the amplified signal toward a mobile station 2 from an antenna. A despreading unit 2a in the receiver of the mobile station applies despread processing to the receive signal and a demodulator 2b demodulates the receive data (step 101). A SIR measurement unit 2c measures the power ratio between the receive signal and an interference signal and a comparator 2d compares target SIR and measured SIR (step 102). If the measured SIR is greater than the target SIR, a TPC (Transmission Power Control) bit generator 2f creates a command that lowers the transmission power by TPC bits (step 103). If the measured SIR is less than the target SIR, on the other hand, the TPC bit generator 2f creates a command that raises the transmission power by the TPC bits (step 104). The target SIR is a SIR value necessary to obtain, e.g., 10−3 (error occurrence at a rate of once every 1000 times). This value is input to the comparator 2d from a target-SIR setting unit 2e. A spread-spectrum modulator 2g spread-spectrum modulates the transmit data and TPC bits. After spread-spectrum modulation, the mobile station 2 subjects the signal to processing such as a DA conversion, orthogonal modulation, frequency conversion and power amplification and transmits the resultant signal toward the base station 1 from an antenna. A despreading unit is on the side of the base station applies despread processing to the signal received from the mobile station 2, and a demodulator 1d demodulates the receive data and TPC bits and controls the transmission power of the base station 1 in accordance with a command specified by the TPC bits (step 105).
FIG. 12 is a diagram showing an uplink frame structure standardized by the 3rd Generation Partnership Project (referred to as “3GPP” below). There is a DPDCH data channel (Dedicated Physical Data Channel) on which only transmit data is transmitted, and a DPCCH control channel (Dedicated Physical Control Channel) on which control data such as a pilot and TPC bit information is multiplexed. After each of these is spread by an orthogonal code, they are mapped onto real and imaginary axes and multiplexed. One frame of the uplink has a duration of 10 ms and is composed of 15 slots (slot #0 to slot #14). The DPDCH data channel is mapped to an orthogonal I channel of QPSK modulation, and the DPCCH control channel is mapped to an orthogonal Q channel of QPSK modulation. Each slot of the DPDCH data channel (I channel) consists of Ndata bits, and the Ndata varies in accordance with the symbol rate. Each slot of the DPCCH control channel (Q channel) that transmits the control data consists of ten bits, has a symbol rate of 15 ksps and transmits a pilot PILOT, transmission power control data TPC, a transport format combination indicator TFCI and feedback information FBI.
The TFCI is a parameter which, when the encoded data of each transport channel TrCH is multiplexed and transmitted on the transmitting side, indicates how the encoded data of each transport channel TrCH was multiplexed in such a manner that the data can be demultiplexed correctly on the receiving side. The TFCI of ten bits is subjected to encoding processing, made a 32-bit TFCI code word and transmitted.
Outer-Loop Transmission Power Control
Owing to changes in traveling velocity during communication and changes in the propagation environment ascribable to travel, the SIR that is necessary to obtain a desired quality (the block error rate, or BLER) is not constant. In order to deal with these changes, the block error rate BLER is observed and control is exercised so as to increase the target SIR if the observed value of BLER is inferior to the target BLER and decrease the target SIR if the observed value of BLER is superior to the target BLER. Control that thus changes the target SIR adaptively in order to achieve the desired quality is well known as outer-loop transmission power control (outer-loop TPC).
FIG. 13 is a block diagram illustrating a transmission power control apparatus on the receiving side, the apparatus including an outer-loop control section. Components identical with those shown in FIG. 10 are designated by like reference characters. FIG. 14 is a processing flowchart of outer-loop control. As shown in FIG. 13, the apparatus includes a radio unit 21, a modem unit 22 and a codec unit 23.
A signal that has been transmitted from the base station 1 is received by the radio unit 21 and is demodulated by the demodulator 2b of the modem unit 22, after which the demodulated signal is decoded by an error-correction decoder 2h of the codec unit 23 (step 201). A receive BLER measurement unit (e.g., a CRC detector) 2j performs CRC error detection for every transport block TrBk and inputs the result (measured BLER) of error detection of each transport block TrBk to a target-SIR controller 2k (step 202).
Since the system specifies target block error rate (BLER) by transport channel (TrCH), the target-SIR controller 2k compares the input measured BLER and the target BLER (step 203), exercises control of the target SIR in such a manner that the target SIR is increased a prescribed amount (step 204) if the measured BLER is greater than the target BLER and decreased a prescribed amount (step 205) if the measured BLER is less than the target BLER. The update period and amount of update of the target SIR change depending upon the target quality specified. By virtue of the control described above, the target SIR is regulated as indicated intervals A, B in FIG. 15.
The outer-loop control set forth above is for a case where the transmitting side is transmitting data on the DPDCH (FIG. 12). However, in a case where data is not being transmitted, the codec unit 23 cannot measure the BLER of the receive data. As a consequence, outer-loop control is not carried out and the target SIR is held constant, as indicated at C in FIG. 15. It is required that the constant value of target SIR be set to a value that will make it possible to maintain synchronism between the transmitting side and the receiving side. This is because the mobile station will stop transmitting unless the synchronism between the transmitting side and the receiving side can be maintained.
Since AMR data and UDI (Unrestricted DIgital) data is continuous data, data always exists on the transport channel (TrCH). However, owing to bursty transmission, there are times when a packet does and does not have TrCH data.
Other Outer-Loop Control Scheme
Besides control for adjusting the target SIR using the block error rate (BLER) after error correction and decoding as described above, there is also a proposed technique (International Laid-Open No. WO97/50197) for estimating BLER from the error rate of the pilot signal and updating the target SIR.
Conventionally, a case (receive data non-existent) where only maintenance of communication synchronism by a pilot signal is being performed by the modem unit 22 (FIG. 13) and a case (receive data exists) where communication of control data and user data by the codec unit 23 is being performed are not distinguished from each other. Consequently, there is no data decoded by the codec unit 23 and, in the interval where synchronism of communication is merely being maintained by the pilot signal, the target SIR cannot be controlled and becomes a constant value (C in FIG. 15). In this interval, however, it is required that enough power for enabling maintenance of synchronism be generated irrespective of the target SIR. As a result, the prior art is such that in an interval in which data is not being transmitted, the difference between the target SIR of a constant value and the smallest SIR necessary to maintain synchronism by the pilot signal becomes excessive power.
Further, with control that updates the target SIR by a pilot signal as disclosed in Patent Reference 1, the target SIR can be updated even when communication is taking place solely on DPCCH, i.e., even when there is no receive data. According to the prior art, however, control is exercised without giving any consideration whatsoever to the SIR value needed to maintain synchronism. As a consequence, the difference between the target SIR and the smallest SIR needed to maintain synchronism by the pilot signal becomes excessive power. Further, according to the prior art, the pilot error rate is measured without executing decoding processing and therefore a problem which arises is that the measured error rate contains an error. This means that control of transmission power cannot be carried out.